Combustion gases of hydrocarbon fuels include roughly 10% water vapor whose latent heat may be substantially recovered when the water vapor is condensed in a secondary heat exchanger. In my said co-pending application, a highly efficient secondary heat exchanger is disclosed, to fit within the air casing of a furnace of the type which provides a substantially vertical path, for room air to which the heat is to be exchanged, between an air blower and a primary heat exchanger. It uses a blower to augment the flow, through a heat exchange coil, of the cooled combustion gases, which afford little draft and whose flow might be further impeded by condensate forming in the three sequential runs of tubing. The second and third runs are inclined downward, requiring them to be separately finned.